In some circumstances it can be desirable to extract mineral values from an underground formation by dissolving the valuable constituents in situ. Insoluble gange minerals can be left in the underground voids or solution cavities in the formation, thereby minimizing disposal problems which follow above ground processing. Such a technique can be suitable for minerals that are essentially water soluble, such as halite or trona, minerals that can be dissolved in alkaline solutions such as nahcolite, or acid soluble minerals such as oxidized copper ore. The soluble minerals can be in massive bodies readily dissolved by circulating leach liquid or can be dispersed in an insoluble matrix, either as veins, stringers, beds, or laminations and pockets, or as phenocrysts of sought after values. Fragmenting can be important for exposing dispersed minerals to leach solutions.
Leaching or solution mining can be conducted on the fragmented material present in stopes following conventional mining operations. Alternatively, an ore to be leached can be explosively expanded following mining operations so as to be sufficiently permeable to permit passage of leaching liquids. These techniques involve underground mining with its associated costs and hazards.
It can be preferable to gain access to the mineral by way of bore holes or wells from the ground surface since no mining operations are required. In such a technique the leaching solution contacts unfragmented formation which can have low permeability and consequently low rates of leaching. It can therefore be desirable to induce and enhance permeability by fragmenting the subterranean formation associated with the cavity and/or mineralized mass, thereby increasing the rate of dissolution and enhancing the rate and effectiveness of leaching. Void space and exposed surface are needed for permeability for better leaching than a bore hole furnishes.
It is therefore desirable to create a cavity adjacent a well bore or interconnecting a plurality of well bores and conduct leaching operations in and around such a cavity. A substantial quantity of subterranean formation can be contacted by leaching solution in such a situation.
An initial cavity for leaching or solution mining can be formed by a variety of techniques. When a plurality of wells are provided in the subterranean formation the cavity can be initiated by hydraulic fracturing, that is, by increasing hydraulic pressure in one or more of such wells until a fracture is induced through the subterranean formation, providing communication with one or more additional wells. The locus of such a hydraulic fracture is sometimes guided by initially "notching" formation adjacent the bore of a well. Notching can be conducted by several methods; for example, a shaped charge of explosive, bullets, or the like, can be used for perforating the well bore and providing a locus for initiation of a fracture. Alternatively, a horizontal slot in the wall of the well bore can be cut by an underreamer.
An initial cavity can be formed by leaching action which can be localized by introducing a hydrocarbon "cap" over the leaching solution. Alternatively, such leaching can be directionally guided with jets of the leaching solution. Such an arrangement can be employed for interconnecting wells in an array of wells or for initiating a cavity adjacent a single well.
An initial cavity can be formed by explosive "springing" or by using a high pressure jet for eroding formation adjacent a well. Springing is a technique for enlarging the bottom of a drill hole by exploding a small explosive charge in it. A number of charges of increasing size can be detonated for gradually increasing the hole size to a desired extent. These techniques have a tendency to create an undesirable vertical dimension of the initial cavity and expose a large surface area.
Connections can also be made between wells by "whipstocking" or angle drilling a new well into the bore of an existing well or a cavity surrounding an old well. A combination of such techniques can be employed for initiating and/or enlarging void space in the subterranean formation for in situ leaching.
It is desirable to continually enlarge the cavity for exposing additional mineral for solution mining or in situ leaching. Leaching alone can gradually enlarge the cavity. The cavity can enlarge by spalling or sloughing of formation from the walls and roof of the cavity into the cavity and by collapse of overlying formation from the roof into the cavity. The walls and roof can be weakened by continual leaching action for enlarging the cavity. Particles of formation sloughing into the cavity can remain in place and the cavity can contain a substantial volume of such permeable rubble which continues to be subjected to leaching action. Some of the smaller insoluble particles can be withdrawn in leach solution withdrawn from the cavity. Such withdrawal can augment solution of the soluble minerals for increasing volume in the cavity for further fragmentation and exposure of minerals to leaching.
Enlargement of such a cavity and exposure of new surface for leaving solely by leaching action and lithostatic forces can be slower than desired and the rate of recovery of mineral values from the formation can thereby be limited. It is desirable to provide techniques for enlarging such a subterranean cavity at a rate faster than accomplished by leaching alone. It is desirable to increase surface area exposed to leaching action and enhance permeability of the formation.